Neptune – This Week & Next

Robert Conrad, our Observing Chair, posted on Facebook about a close conjunction of Neptune with the 4.2 magnitude star Phi Aquarii in Aquarius. The pair will be less than 15 arc-seconds apart on Thursday, Sept 6th – that is about a third of Jupiter’s apparent size at opposition – so the pair will appear practically on top of each other.

The Great Dark Spot of Neptune at the top accompanied by “Dark Spot 2” further south. Image credit: NASA/Voyager 2 Team

But then just after midnight on Monday, Sept 9th (technically, it will be Tuesday at 00:07:12 am), Neptune reaches opposition when it is directly opposite the Sun as viewed from the Earth. Neptune will have moved slightly to the west of Phi Aquarii by then but is still close – within 10 arc=minutes.

If you haven’t seen Neptune then this is a great opportunity. Neptune is not visible to the naked-eye as its magnitude of 7.8 is well past the limit for naked-eye observations. You may get a glimpse of it using steady-supported binoculars but a 200x view through a 150 mm or larger telescope is required to resolve it into a disk. Either way, the 4.2 magnitude star Phi Aquarii is a good guide. AAVSO charts are on Robert’s post at:

Even better is to observe Neptune over several nights and notice its motion relative to Phi Aquarii. Neptune’s orbital period of 164.6 years makes it move slowly across the sky, it will still be together with Phi Aquarri in a one degree field of view on Oct 1st, 2019.

Recording the relative positions over several nights lets you avoids Galileo’s missed opportunity – there is evidence that Galileo observed Neptune on January 6th, 1613, and again on January 27, 1613 and noted a slight discrepancy in its position versus the background stars. However, there is no record that he made further observations and he likely thought it to be a fixed blue star rather than a planet.

Credit for Neptune’s discovery goes to Britain’s John Couch Adams and France’s Urbain Le Verrier who had worked out the position of a theoreticl 8th planet independently based on perturbations in the observed orbit of Uranus. Le Verrier’s analysis predicted the new planet’s location to with one degree of where it was observed by J. G. Galle and H. L. d’Arrest, staff astronomers at the Berlin Observatory, in 1846.

Neptune is a gas giant, like its near twin Uranus: it has more mass than Uranus but is slightly smaller because its greater mass cause more gravitational compression of its atmosphere. The methane in Neptune’s upper atmosphere absorbs the red light from the Sun but reflects the blue light from the Sun back into space. This is why Neptune appears blue. Neptune has the strongest winds of any planet in our solar system with wind speeds reaching 2,000 km/h, three time stronger than Jupiter’s. It has several large dark spots with the largest known as the Great Dark Spot – similar to the hurricane-like storms and the Great Red Spot on Jupiter.